Union officials have said a Tube strike at Christmas is possible, as workers are due to be balloted in a jobs row.
The dispute involves Tube maintenance workers
About 1,800 RMT (Rail, Maritime and Transport) union members working for Tube maintenance firm Metronet will be asked to strike.
An RMT spokesman said given the time scale of proceedings a walkout at Christmas "could not be ruled out".
Metronet said action was unwarranted and would cause huge disruption in the run-up to Christmas if it went ahead.
Meanwhile, Tube drivers may be asked to take action because the RMT said, if Metronet workers go out, the tracks may not be properly inspected.
Bosses at Metronet were given notice of the ballot on Thursday before it goes ahead in about seven days time.
The RMT blames Metronet, which maintains nine Underground lines, for planning to subcontract work and make "front line" staff redundant.
"We made it clear to Metronet that its plans to subcontract work and further fragment the Tube's infrastructure were unacceptable to us - particularly after the recent brake failures on the Northern Line," said RMT general secretary Bob Crow.
But Metronet said plans to use subcontractor Bombardier to supply and maintain new trains had been known for a long time.
It has announced plans to make up to 285 people redundant but said more than half would be agency staff and the rest would be voluntary redundancies.
The spokesman added: "The RMT have an unflattering record of calling for industrial action in the month of December - on no less than seven occasions since 1996 - and all of which were called off."
The RMT also said London Underground (LU) had failed to persuade them that, should the strike go ahead, full train and track inspections would be maintained.
As a result, 1,500 drivers will be asked to vote for action "short of a strike".
LU has said the dispute is between the RMT and Metronet and said the RMT should wait for the result of the Metronet ballot, before threatening action among Tube drivers.